The present invention generally relates to a sensor for nondestructive inspection of magnetically permeable objects such as wire cables, rods, pipes and the like. The invention is concerned more particularly with an eddy current sensor assembly for detecting structural faults in the objects.
Eddy current (EC) devices are known for inspecting elongated magnetically permeable objects such as wire cables, rods, pipes, and the like for structural defects such as stress-corrosion cracks. One such device shown in my U.S. Pat. No. 5,751,144, the disclosure of which is herein incorporated by reference, includes magnet means which induces a magnetic flux which magnetically saturates the object along a longitudinal section. The magnet means and the induced magnetic flux move progressively and longitudinally relative to the object whereby a longitudinal section of the object experiences a changing magnetic flux which induces eddy currents. An eddy current change detecting means is provided on the magnet means and is to be positioned adjacent to the relatively moving object to detect changes in eddy currents which are representative of structural faults in the object.
Eddy current inspection and sensing methods for faults in objects made of ferromagnetic materials suffer from two critical problems. First, the high permeability of the ferromagnetic material acts as a shield because of the frequency-permeability-conductivity term that appears in skin depth calculations. Thus, full-wave penetration of the object wall is difficult to achieve. Second, the permeability, coercive force and remanence of steel are influenced by material properties, by internal stresses and by structural conditions. These magnetic influences depend on the selection of the initial materials and the melting, foundry, rolling and annealing processes. Because these magnetic characteristics are not well controlled during manufacture and handling, magnetic properties can vary in a random fashion along the length of the elongated object. Localized permeability variations, in the absence of auxiliary magnetization, usually lead to noise levels that prevent sufficiently high sensitivities during testing.
There are three ways to increase the through-wall penetration depth and the signal-to-noise ratio of EC inspections. First, the object can be magnetically saturated to decrease its magnetic permeability and, thereby, increase skin depth. Magnetic saturation also decreases localized permeability variations, which in turn decreases distortions of the inspection signals, thereby improving the signal-to-noise ratios. Second, the skin depth can be increased by lowering the excitation frequency. And, third, the strength of the excitation signal can be increased.
For through-wall inspections, it is therefore necessary to magnetically saturate the material to be inspected by a DC magnetic field. When ferromagnetic material is magnetically saturated, its relative permeability approaches a value of one (i.e., that of air). When thus saturated, the material behaves like a non-ferromagnetic material and permeability variations will not affect the EC inspection. The low relative permeability decreases the background noise and improves the signal-to-noise ratio so that discontinuity signals can be sensed. In addition to noise reduction, the DC magnetization method decreases the skin effect which is otherwise problematic when applying an alternating magnetic field associated with conventional eddy current methods.
Feasibility experiments have shown that a simple DC magnetic saturation method is easily implemented. However, for a reliable through-wall detection of structural faults such as axial slits, two necessary conditions must be simultaneously satisfied. First, the magnetically permeable elongated object must be magnetically saturated or nearly saturated. Second, significant eddy currents must be induced so that eddy current changes representative of structural faults can be readily detected. The previous two conditions will hereinafter be referred to as the "necessary conditions".
Unfortunately, the necessary conditions are somewhat incompatible with each other because the magnet means moving relative to the tested object induces a changing magnetic field which, according to Lenz's law, excites eddy currents together with an associated magnetic field that opposes the change in the magnetic field which produced these eddy currents. Therefore, the induction of eddy currents retards the diffusion of the magnetic flux through the object wall so as to oppose magnetic saturation of the pipe. These motion induced eddy currents will be called Self-Excited Eddy Currents (SEECs) hereafter. Full magnetic saturation (or near saturation) is achieved only after the eddy currents (the SEECs) have decayed toward zero magnitude. In other words, for a simple implementation of an SEEC method, the independent control of the magnetic saturation together with the simultaneous induction of strong SEECs is not possible. As such, prior SEEC apparatus and methods suffer from the difficulty of simultaneously achieving the necessary conditions for reliable structural fault detection.
A solution for improving the reliability of structural fault detection is to provide a magnetic inspection device having two primary and opposite poles which induce a magnetic flux to place the object at least near magnetic saturation. At least one auxiliary pole is positioned on the inspection device between the primary poles and serves to boost the level of the magnetic flux induced by the primary poles so as to strengthen induced eddy currents during relative movement of the inspection device and the object. At least one sensor positioned on the inspection device detects eddy current changes which are representative of structural faults. A drawback with current auxiliary pole methods for inducing eddy currents is that the eddy current sensors and auxiliary poles are lifted off of the pipe wall when the magnetic inspection device contacts a bead of a girth weld or longitudinal weld connecting adjacent pipe sections. The lifting of the eddy current sensor and auxiliary poles cause a distortion in the readings of the eddy current sensor which can cause a structural fault to be undetected if a structural fault is located near or at a girth weld.
Accordingly, it is a general object of the present invention to provide an eddy current sensor assembly that eliminates or otherwise minimizes distortion in detecting changes in eddy currents when a magnetic inspection device contacts a girth weld or other projection or obstruction in an inspected pipe wall.